1. Field
This disclosure relates to electrical discharge machining (EDM).
2. General Background
Single-use fasteners, such as a rivet or aerospace fastener where the fastener is formed during the assembly process, is common in many industries. In aerospace, the maintenance of air frames frequently requires removal of hundreds of fasteners in order to replace or repair structural members such as longerons, bulkheads, center barrels, and the like. Fasteners commonly include rivets or threaded fasteners which have been malleably distorted so they cannot be directly removed. Many of these fasteners are manufactured in titanium or other difficult to machine materials.
For example, during assembly, an extended portion often referred to as a “shank” of a fastener extends through one or more frames and may be pulled with respect to a collar on the opposite side. When the appropriate tension in the fastener shank is achieved, the collar may be malleably crushed against the shank of the fastener to form a permanent structure. Removal of the fastener and collar from the frame(s) may be challenged due to the fixation of the fastener and the collar, the toughness of the materials used for fasteners and collar, and the delicate condition of surrounding frames to which they are applied.
Traditional methods which have been utilized for many years to remove such fasteners is to machine away the head of the fastener with a drill which is manually positioned. The drill adds pressure to the region being drilled as well as heat. Additionally, when the fastener is of titanium or other difficult to machine materials, such drilling results in a significant consumption of drill bits. Traditional drilling of fasteners operation has a known risk of damage to the structure in which the fastener is engaged. Damage to surrounding structures may result from vibration, drill bit slips, or drilling too deep. If drill bit slippage damage occurs to the surrounding material, or if the hole should be cut too deep, an oversized fastener might be used during reassembly, or the entire component may need to be replaced. Measures to correct such undesirable damage may result in additional expense associated with the operation.
In some cases, safety regulations and guidelines indicate a maximum number of errors that can be diagnosed and treated before an entire portion of a workpiece must be abandoned and replaced. Some regulations and guidelines may also require inspections of areas which appear to have had drill damage to assess the proper corrective action, if any. Such diagnostics require time, may cause delay, and may result in costs.
Electric discharge machining, or EDM, is an established method and apparatus utilized for machining metal. It operates through the utilization of an electrical discharge to remove metal from the workpiece. In the EDM process, an electrode is brought into close proximity to the workpiece. High voltage is applied in pulses at high frequency. The process occurs in the presence of a dielectric fluid. This creates sparking at generally the closest position between the workpiece and the electrode. Particles are removed from the workpiece when sparking is quenched. The duration of the spark (on-time) and the recovery time (off-time) are controlled so that the workpiece and electrode temperatures are not raised to the temperature of bulk melting. Therefore, erosion is essentially limited to a vaporization process.